Kill the lights
After you venture down the hole in the building, you should get to an old subway after you explore the sewers. Once you kill all of the men down there, do not go through the door. Keep going strait through and up a catwalk. Before people in the subway notice you there, tell your men to hold their fire. At the bottom of the catwalk on the other side, you will see a yellow generator. Shoot it, and it should kill the lights. Put on your night vision goggles but be careful; they will still shoot at you but will usually miss. Tell your men to open fire and move through the subway to your next checkpoint.

Tank in courtyard
Later in the game you will encounter a courtyard. Take out all the snipers and kill the remaining enemies. After you kill all of them, a red truck with a 50 cal on it will approach from the road ahead. After taking out the .50 cal, go across the street to the library and into the computer store. Looking through the window, throw a smoke grenade at the tank. The tank now cannot see you. Get in position and take out your M203 or grenades and start firing. It takes about four grenades; do not wait too long or the smoke will wear off, and the enemy will open fire.